Method of heat-treating nitride compound semiconductor layer and method of producing semiconductor device

ABSTRACT

A method of heat-treating a nitride compound semiconductor layer, comprising heating a nitride compound semiconductor layer doped with a p-type impurity at a temperature that is at least 200° C. but less than 400° C. for at least 100 minutes.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

[0001] The present invention relates to a method of heat-treating anitride compound semiconductor layer and a method of producing asemiconductor device.

[0002] In recent years, gallium nitride compound semiconductors such asGaN, an AlGaN compound crystal and an AlInGaN compound crystal areconsidered promising as component materials for semiconductor devicesthat can emit light in the range of from a visible light region to anultraviolet light region. Particularly, since a light emitting diode(LED) using a gallium nitride compound semiconductor has come to becommercially used, the gallium nitride compound semiconductor deviceshave been of great interest. Further, a semiconductor laser (laserdiode, LD) using a gallium nitride compound semiconductor has beenreportedly realized as well, and applications thereof including a lightsource for an optical disk are expected.

[0003] When a gallium nitride compound semiconductor layer doped with ap-type impurity is formed by a vapor deposition method, the galliumnitride compound semiconductor layer formed by such a method does notbecome a p-type, but it constitutes a semi-insulating layer having ahigh resistance of at least 10⁸ Ω·cm or more, i.e., an i-type compoundsemiconductor layer.

[0004] There is known a method of decreasing the resistance of the abovei-type compound semiconductor layer having a high resistance to convertit to a p-type compound semiconductor layer, as is disclosed, forexample, in JP-A-2-257679. In the method disclosed in JP-A-2-257679, thesurface of an i-type gallium nitride compound semiconductor layerobtained by doping the layer with Mg as a p-type impurity is irradiatedwith electron beam to decrease the resistance of the surface of thegallium nitride compound semiconductor layer. In the above method,however, it is only of the surface of the gallium nitride compoundsemiconductor layer that the resistance can be decreased, and the abovetreatment takes a long time due to scanning with electron beam. Further,there is involved a problem that it is difficult to uniformly decreasethe resistance of the gallium nitride compound semiconductor layer in awafer plane.

[0005] Published Japanese Patent No. 2540791 discloses a technique ofgrowing a gallium nitride compound semiconductor doped with a p-typeimpurity by a vapor deposition method, and heat-treating thesemiconductor at a temperature of 400° C. or higher or at a temperatureof 600° C. or higher for attaining a practical carrier concentration.The above heat-treatment is carried out in vacuum or an inert gasatmosphere free of hydrogen atoms of NH₃ or H₂ for approximately 10 to20 minutes.

[0006] In production of a semiconductor laser (LD), however, diffusionof a p-type impurity such as Mg is presumably more liable to take place,or the sharpness of an interface in a superlattice structure ispresumably more liable to be disintegrated by the diffusion of In withan increase in the heat treatment temperature, and the deterioration ofan active layer such as an increase in threshold voltage I_(th) and adecrease in lifetime is liable to proceed easily.

[0007] Further, when the heat-treatment is carried out at a hightemperature, deterioration takes place in the surface of the galliumnitride compound semiconductor layer due to dissociation of nitrogenatoms. For preventing the above phenomenon, Published Japanese PatentNo. 2540791 also discloses a technique for forming a cap layer on thesurface of a gallium nitride compound semiconductor layer. However, thematerial for constituting the cap layer is selected from Ga_(x)Al_(1−x)N(0≦x≦1), AlN, Si₃N₄ or SiO₂, and finally, it is required to remove thecap layer made of such a material from the surface of the galliumnitride compound semiconductor layer, which results in a problem thatthe number of manufacturing steps increases.

OBJECT AND SUMMARY OF THE INVENTION

[0008] It is therefore an object of the present invention to provide amethod of heat-treating a nitride compound semiconductor layer in whichthe resistance of a nitride compound semiconductor layer doped with ap-type impurity can be decreased, and the nitride compound semiconductorlayer can be activated, at a far lower temperature than a temperatureemployed by any conventional method, and a method of producing asemiconductor device by applying the above method of heat-treating anitride compound semiconductor layer.

[0009] According to a first aspect of the present invention forachieving the above object, there is provided a method of heat-treatinga nitride compound semiconductor layer, which method comprises heating anitride compound semiconductor layer doped with a p-type impurity at atemperature that is at least 200° C. but less than 400° C., preferablythat is at least 225° C. but less than 400° C., more preferably that isat least 250° C. but less than 400° C., still more preferably that is atleast 300° C. but less than 400° C., for at least 100 minutes,preferably for at least 200 minutes, more preferably for at least 500minutes, still more preferably at least 20 hours, yet more preferablyfor at least 30 hours, far more preferably for at least 3×10³ minutes(50 hours), further more preferably for at least 1×10² hours.

[0010] According to a first aspect of the present invention forachieving the above object, there is also provided a method of producinga semiconductor device, which method includes the step of heating anitride compound semiconductor layer, which step comprises heating anitride compound semiconductor layer doped with a p-type impurity at atemperature that is at least 200° C. but less than 400° C., preferablythat is at least 225° C. but less than 400° C., more preferably that isat least 250° C. but less than 400° C., still more preferably that is atleast 300° C. but less than 400° C., for at least 100 minutes,preferably for at least 200 minutes, more preferably for at least 500minutes, still more preferably at least 20 hours, yet more preferablyfor at least 30 hours, far more preferably for at least 3×10³ minutes(50 hours), further more preferably for at least 1×10² hours.

[0011] According to a second aspect of the present invention forachieving the above object, there is provided a method of heat-treatinga nitride compound semiconductor layer, which method comprisesheat-treating a nitride compound semiconductor layer doped with a p-typeimpurity in a state where the heating time period t (unit: minute) andthe heating temperature T (unit: K) satisfy conditions of t≧100,preferably t≧200, more preferably t≧500, still more preferably t≧2×10³and the following equation (1),

T≧α/[ln({square root}{square root over (t)})+ln(D ₀)−ln(C)]  (1)

[0012] wherein α is a coefficient that is 1.04×10⁴, ln(D₀) is acoefficient that is 53 and C is a carrier concentration (unit: cm⁻³) ofthe nitride compound semiconductor layer after the heat treatmentthereof.

[0013] According to a second aspect of the present invention forachieving the above object, there is also provided a method of producinga semiconductor device, which method includes the step of heat-treatinga nitride compound semiconductor layer doped with a p-type impurity in astate where the heating time period t (unit: minute) and the heatingtemperature T (unit: K) satisfy conditions of t≧100, preferably t≧200,more preferably t≧500, still more preferably t≧22×10³ and theabovementioned equation (1).

[0014] In the method of heat-treating a nitride compound semiconductorlayer or the method of producing a semiconductor device according to thesecond aspect of the present invention (these methods will be sometimesgenerally referred to as “method according to the second aspect of thepresent invention” hereinafter), desirably, the heating temperature T(K) is brought into a state where the condition of 473 (K)≦T<673 (K),preferably 498 (K)≦T<673 (K), more preferably 523 (K)≦T<673 (K), stillmore preferably 573 (K)≦T<673 (K) is satisfied. When the heatingtemperature T is expressed in terms of degree Celsius, the heatingtemperature is desirably a temperature that is at least 200° C. but lessthan 400° C., preferably that is at least 225° C. but less than 400° C.,more preferably that is at least 250° C. but less than 400° C., stillmore preferably that is at least 300° C. but less than 400° C. Further,the carrier concentration of the nitride compound semiconductor layerafter the heat treatment is at least 1.0×10¹⁷ cm⁻³, preferably at least3.0×10¹⁷ cm⁻³, more preferably 5.0×10¹⁷ cm⁻³, still more preferably1.0×10¹⁸ cm⁻³.

[0015] In the method of heat-treating a nitride compound semiconductorlayer or the method of producing a semiconductor device according to thefirst aspect of the present invention (these methods will be sometimesgenerally referred to as “method according to the first aspect of thepresent invention” hereinafter) or the method according to the secondaspect of the present invention, the heating atmosphere can be an aerialatmosphere (having a pressure that may be atmospheric pressure, areduced pressure or an elevated pressure). Alternatively, the heatingatmosphere can be an atmosphere supplied at least with an oxygen gas,and in this case, the heating atmosphere may be an atmosphere suppliedwith an oxygen gas alone, an atmosphere supplied with an oxygen gas anda hydrogen gas, an atmosphere supplied with an oxygen gas and steam, oran atmosphere supplied with an oxygen gas, a hydrogen gas and steam.Further, the heating atmosphere may be an atmosphere supplied with aninert gas in addition to these. Alternatively, the heating atmospheremay be an inert gas atmosphere or a reduced-pressure atmosphere having apressure lower than atmospheric pressure, and in this case, the heatingatmosphere may contain steam. The inert gas can be selected fromnitrogen (N₂) gas, helium (He) gas, neon (Ne) gas, argon (Ar) gas, ormixtures of these gases. When an oxygen gas and a hydrogen gas aresupplied, or when an oxygen gas, a hydrogen gas and steam are supplied,the amount ratio of the hydrogen gas in a mixture of the oxygen gas andthe hydrogen gas is required to be less than the lower limit (4% byvolume) of a combustion range. The supply ratio of the oxygen gas/steamand the amount ratio of the steam in the inert gas atmosphere or thereduced-pressure atmosphere are essentially arbitrary.

[0016] According to a third aspect of the present invention forachieving the above object, there is provided a method of heat-treatinga nitride compound semiconductor layer, which method comprises heating anitride compound semiconductor layer doped with a p-type impurity, at atemperature that is at least 200° C. but not, higher than 1200° C., inone atmosphere selected from;

[0017] (A) aerial atmosphere,

[0018] (B) an atmosphere supplied with an oxygen gas and a hydrogen gas,

[0019] (C) an atmosphere supplied with an oxygen gas and steam,

[0020] (D) an atmosphere supplied with an oxygen gas, a hydrogen gas andsteam,

[0021] (E) an inert gas atmosphere containing steam, or

[0022] (F) a reduced-pressure atmosphere containing steam and having apressure lower than atmospheric pressure.

[0023] According to a third aspect of the present invention forachieving the above object, there is also provided a method of producinga semiconductor device, which method includes the step of heating anitride compound semiconductor layer doped with a p-type impurity, at atemperature that is at least 200° C. but not higher than 1200° C., inone atmosphere selected from;

[0024] (A) aerial atmosphere,

[0025] (B) an atmosphere supplied with an oxygen gas and a hydrogen gas,

[0026] (C) an atmosphere supplied with an oxygen gas and steam,

[0027] (D) an atmosphere supplied with an oxygen gas, a hydrogen gas andsteam,

[0028] (E) an inert gas atmosphere containing steam, or

[0029] (F) a reduced-pressure atmosphere containing steam and having apressure lower than atmospheric pressure.

[0030] In the method of heat-treating a nitride compound semiconductorlayer or the method of producing a semiconductor device according to thethird aspect of the present invention (these methods will be sometimesgenerally referred to as “method according to the third aspect of thepresent invention” hereinafter), the amount ratio of the hydrogen gas ina mixture of the oxygen gas and the hydrogen gas in the atmospheresupplied with an oxygen gas and a hydrogen gas or the atmospheresupplied with an oxygen gas, a hydrogen gas and steam is required to beless than the lower limit (4% by volume) of a combustion range. Further,the supply ratio of the oxygen gas/steam in the atmosphere supplied withan oxygen gas and a hydrogen gas and the amount ratio of the steam inthe inert gas atmosphere or the reduced-pressure atmosphere areessentially arbitrary. Further, the above atmospheres (A) to (E) may bein a state having any one of atmospheric pressure, a reduced pressureand an elevated pressure. Further, the above atmospheres (B), (C) and(D) may contain the above inert gas.

[0031] In the method according to the third aspect of the presentinvention, desirably, the lower limit of the heating temperature is atleast 200° C., preferably at least 225° C., more preferably at least250° C., still more preferably at least 300° C. Desirably, the upperlimit of the heating temperature is 1200° C. or lower, preferably 700°C. or lower, more preferably 600° C. or lower, still more preferably500° C. or lower, yet more preferably less than 400° C. When the upperlimit of the heating temperature is set at 700° C. or lower, thesharpness of an interface in a superlattice structure is less liable tobe disintegrated by the diffusion of atoms (for example, In)constituting the nitride compound semiconductor layer. Further, when theupper limit of the heating temperature is set at 600° C. or lower,further at 500° C. or lower, further at a temperature lower than 400°C., the dissociation of nitrogen atoms from the nitride compoundsemiconductor layer can be more reliably prevented, and the diffusion ofa p-type impurity such as Mg, etc., is less liable to take place.Further, the surface of the nitride compound semiconductor layer is lessliable to be oxidized although its degree differs depending upon anatmosphere employed for the heat-treatment.

[0032] In the method according to the first, second or third aspect ofthe present invention, there may be employed a constitution in which ahydrogen-permeable film is formed on the surface of the nitride compoundsemiconductor layer. In this constitution, examples of a material forconstituting the hydrogen-permeable film include so-calledhydrogen-occlusion metals such as palladium (Pd) and hydrogen-occlusionalloys. The thickness of the hydrogen-permeable film is not speciallylimited so long as the dissociation of nitrogen atoms from the nitridecompound semiconductor layer by the heat treatment can be prevented. Thehydrogen-permeable film can be formed by a physical vapor depositionmethod (PVD method) such as a sputtering method and a vacuum depositionmethod or a chemical vapor deposition method (CVD method). For example,a hydrogen-permeable film made of palladium (Pd) is permeable tohydrogen gas at a high temperature, so that it can release hydrogenatoms in the nitride compound semiconductor layer to the heat-treatmentatmosphere and can prevent oxidation of the surface of the nitridecompound semiconductor layer. Further, palladium can be easily peeledoff the nitride compound semiconductor layer and can be also used as ap-side electrode, so that it does not cause the number of steps in theprocess of producing a semiconductor device, etc., to increase much ascompared with the cap layer disclosed in Published Japanese Patent2540791.

[0033] The nitride compound semiconductor layer in the present inventionspecifically includes GaN, an AlGaN compound crystal, an AlInGaNcompound crystal, a BAlInGaN compound crystal, an InGaN compoundcrystal, InN and AlN. And, it can be formed by a metal organic chemicalvapor deposition method (MOCVD method) or a molecular beam epitaxialmethod (MBE method). The p-type impurity includes Mg, Zn, Cd, Be, Ca, Baand O.

[0034] In the method of producing a semiconductor device according tothe first, second or third aspect of the present invention, thesemiconductor device includes a semiconductor laser (laser diode, LD), alight-emitting diode (LED) and a transistor such as HBT.

[0035] The above heat treatment can be carried out, for example, with anelectric oven, various heating apparatuses including heating apparatusesusing hot gases such as a hot-air heating apparatus, or an apparatus forirradiation with light or electromagnetic wave such as infrared beam,ultraviolet beam or microwave.

[0036] In the method according to the first aspect of the presentinvention, the heating temperature is set at a temperature that is atleast 200° C. but less than 400° C., namely lower than that employed inany conventional method, and the heat treatment is carried out for alonger period of time than that employed in any conventional method,whereby the resistance of the nitride compound semiconductor layer canbe decreased and the nitride compound semiconductor layer can beactivated. In the method according to the second aspect of the presentinvention, the heat treatment is carried out under the conditions wherethe heating temperature T and the heating time period t satisfy t≧100andthe equation (1), whereby the resistance of the nitride compoundsemiconductor layer can be reliably decreased and the nitride compoundsemiconductor layer can be reliably activated. In the method accordingto the third aspect of the present invention, the heat treatment iscarried out in the atmosphere containing an oxygen gas or containingsteam, so that the lower limit of the heating temperature can bedecreased as compared with a conventional method. Further, in the methodaccording to the first, second or third aspect of the present invention,the heating atmosphere is selected from aerial atmosphere or anatmosphere supplied at least with an oxygen gas, so that the resistanceof the nitride compound semiconductor layer can be decreased and thenitride compound semiconductor layer can be activated for a shortheating time period. This is presumably because water, for example,contained in aerial atmosphere works as a kind of catalyst, or oxygenworks as a kind of catalyst, on the surface of the nitride compoundsemiconductor layer during the heat treatment so that the water oroxygen promotes dissociation of hydrogen in the nitride compoundsemiconductor layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The present invention will be explained with reference toExamples with referring to drawings hereinafter.

[0038]FIG. 1 is a graph showing carrier concentrations based on holecoefficient measurements at heating temperatures T of 385° C., 415° C.and 485° C.

[0039]FIG. 2 is a graph showing electric resistance values at heatingtemperatures T of 385° C., 415° C. and 485° C.

[0040]FIG. 3 is a graph showing a relationship between the heatingtemperature T and the heating time period t when the carrierconcentration C of a nitride compound semiconductor layer after heattreatment is varied.

[0041]FIG. 4 shows a schematic cross-sectional view of a semiconductorlaser (laser diode, LD).

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

[0042] Example 1 is concerned with the methods of heat-treating anitride compound semiconductor layer according to the first, second andthird aspects of the present invention. Example 1 employed an aerialatmosphere (pressure: atmospheric pressure) as a heating atmosphere.

[0043] In Example 1, first, a 40 nm thick buffer layer was formed on asapphire substrate, then, a 1 μm thick non-doped GaN layer thatcontained no impurity was formed thereon, and further, a 1 μm thicknitride compound semiconductor layer made of GaN containing Mg as ap-type impurity was formed on the GaN layer. These layers were formed byan MOCVD method. The thus-obtained sample was cut into pieces havingdimensions of 5 mm×5 mm, and 0.3 μm thick electrodes made of palladium(Pd) were formed on four corners by a vapor deposition method, toprepare test samples for heat-treatment evaluation.

[0044] The above test samples for heat-treatment evaluation were placedon hot plates made of stainless steel at T ° C. (specifically, 385° C.,415° C., 485° C.), weights made of stainless steel were placed on thetest samples to increase adhesion between the test samples forheat-treatment evaluation and the hot plates. Aerial atmosphere (averagetemperature 28° C., average relative humidity 68%) was employed as aheating atmosphere. During the heat treatment, hydrogen was releasedfrom the test samples through vary small gaps between the hot plates andthe test samples.

[0045] After a predetermined period of time, each test sample forheat-treatment evaluation was removed from the hot plate and measuredfor an electric resistance and a hole coefficient according to a van derPauw method. Then, the test samples were again placed on the hot platesto continue the heat treatment.

[0046] In addition, a test sample for heat-treatment evaluation,prepared by forming a 0.3 μm thick hydrogen-permeable film made ofpalladium (Pd) on a 1 μm thick nitride compound semiconductor layer madeof GaN containing Mg as a p-type impurity, was also measured for anelectric resistance and a hole coefficient. Concerning the test samplein this case, the hydrogen-permeable film corresponding to theelectrodes was retained and other portions of the hydrogen-permeablefilm were removed by etching before the measurement of an electricresistance and a hole coefficient.

[0047]FIGS. 1 and 2 show the electric resistance values at T=385° C.,415° C. and 485° C. and the carrier concentrations based on results ofthe hole coefficient measurement. In the graph of FIG. 1, the axis ofabscissas shows square roots of heating time period t (unit: minute),and the axis of ordinates shows the carrier concentrations. In the graphof FIG. 2, the axis of abscissas shows square roots of heating timeperiod t (unit: minute), and the axis of ordinates shows results of theelectric resistance measurement. In FIG. 1, solid square marks show thecarrier concentrations at T=385° C., blank square marks show the carrierconcentrations at T=415° C., and blank circles show the carrierconcentrations at T=485° C. Further, in FIG. 2, solid square marks showthe electric resistance values at T=385° C., blank square marks show theelectric resistance values at T=415° C., and blank circles show theelectric resistance values at T=485 °C. Further, a solid circle in FIG.1 and a solid circle in FIG. 2 show the carrier concentration and theelectric resistance value of the test sample having thehydrogen-permeable film at T=385° C. FIGS. 1 and 2 show the following.Even if the heating temperature is lower than 400° C., the carrierconcentration increases and the electric resistance decreases with anincrease in the heating time period. The measurement results of the testsample having the hydrogen-permeable film are poor as compared with themeasurement results of the test samples having no hydrogen-permeablefilm. This is presumably because the diffusion rate of hydrogendecreases due to the hydrogen-permeable film.

[0048] The results shown in FIG. 1 show that the carrier concentration Cis nearly in proportion to the square root of the heating time period t.It is assumed from this result that an increase in the carrierconcentration, i.e., activation proceeds on the basis of diffusion, andit can be assumed that the increase in the carrier concentration isaccording to the following equation (2), wherein T is a heatingtemperature (unit: K).

C=D ₀ ·{square root}{square root over (t)}·exp[−α/T]  (2)

[0049] When it is assumed that the carrier concentrations C₁, C₂ and C₃at the heating temperatures 385° C., 415° C. and 485° C. are inproportion to the square root of the heating time period t, thecoefficients D₁, D₂ and D₃ at the heating temperatures 385° C., 415° C.and 485° C. are determined as follows.

C ₁ =D ₁ {square root}{square root over (t)}  (3-1)

C ₂ =D ₂ {square root}{square root over (t)}  (3-2)

C ₃ =D ₃ {square root}{square root over (t)}  (3-3)

[0050] As a result, the following results were obtained.

D ₁=1.39×10¹⁶

D ₂=3.61×10¹⁶

D ₃=1.17×10¹⁷

[0051] On the basis of the above results, the coefficients D₀ and α aredetermined using the following equation (4). The above values of D₁, D₂and D₃ are used as values of D. As a result, the coefficient ln(D₀)=53and α=1.04×10³ are obtained.

D=D ₀˜exp[−α/T]  (4)

[0052] The equation (2) is modified to give the following equation (5).When the heating temperature T on the left-side member of the equation(5) is greater than the right-side member, therefore, a desired carrierconcentration can be obtained after the heat treatment.

T=α/[ln({square root}{square root over (t)})+ln(D ₀)−ln(C)]  (5)

[0053]FIG. 3 shows a graph obtained by varying the carrier concentrationC (unit: cm⁻³) of the nitride compound semiconductor layer after theheat treatment in equation (5). In FIG. 3, the axis of ordinates showsthe heating temperature T (unit: ° C.), and the axis of abscissas showsthe heating time period (unit: minute). In FIG. 3, solid rhombic marksshow a case where C=1×10¹⁶ cm⁻³, solid square marks show a case whereC=5×10¹⁶ cm⁻³, solid triangular marks show a case where C=1×10¹⁷ cm⁻³,“x” marks show a case where C=3×10¹⁷ cm⁻³, “*” marks show a case whereC=5×10¹⁷ cm⁻³, and solid circular marks show a case where C=1×10¹⁸ cm⁻³.

[0054] The following Tables 1 and 2 show relationships between theheating time period t and the carrier concentration C of the nitridecompound semiconductor layer after the heat treatment when the heatingtemperature is set, for example, at 400° C. and 385° C. The heating timeperiod t of 41 hours or 82 hours is not any critical time period in viewof the production of a semiconductor device. TABLE 1 Heating temperatureT = 400° C. Carrier concentration Heating time period C (cm⁻³) t (hour)3 × 10¹⁷ 3.7 5 × 10¹⁷ 10 1 × 10¹⁸ 41

[0055] TABLE 2 Heating temperature T = 385° C. Carrier concentrationHeating time period C (cm⁻³) t (hour) 3 × 10¹⁷ 7.4 5 × 10¹⁷ 21 1 × 10¹⁸82

[0056] The above experiments were repeated in the same manner as in theabove-described Example 1 except that the aerial atmosphere as aheat-treatment atmosphere was replaced with an atmosphere supplied withan oxygen gas, an atmosphere supplied with an oxygen gas and a hydrogengas, an atmosphere supplied with an oxygen gas and steam, an atmospheresupplied with an oxygen gas, a hydrogen gas and steam, an inert gasatmosphere, an inert gas atmosphere containing steam, a pressure-reducedatmosphere and a pressure-reduced atmosphere containing steam. As aresult, a tendency similar to that obtained in the aerial atmosphere wasobtained.

EXAMPLE 2

[0057] Example 2 is concerned with the method of producing asemiconductor device that is a semiconductor laser (LD). The method ofthe above production according to a pressurized MOCVD method that is akind of a vapor deposition method will be outlined below. In apressurized MOCVD method, desirably, the MOCVD apparatus for formingvarious compound semiconductor layers has an internal pressure of 1.1 to2.0 atmospheric pressures, preferably 1.2 to 1.8 atmospheric pressures.When such a pressurized MOCVD method is employed, there can be reliablyprevented a phenomenon that nitrogen is dissociated during the growth ofa compound semiconductor layer to cause deficiency of nitrogen in thecompound semiconductor layer. In the following explanation, the pressurein the MOCVD apparatus was set at 1.2 atmospheric pressures when variouscompound semiconductor layers were formed. The pressure in the MOCVDapparatus may be atmospheric pressure. Further, the temperature forforming various compound semiconductor layers other than a buffer layerand an active layer was set at approximately 1000° C., the temperaturefor forming the active layer was set at 700to 800° C. for preventingdecomposition of In, and the temperature for forming the buffer layerwas set at approximately 560° C.

[0058] First, a sapphire substrate 10 is introduced into an MOCVDapparatus (not shown), and the MOCVD apparatus is deaerated. Then, whilehydrogen gas is flowed, the sapphire substrate 10 is heated to removeoxide on the surface of the sapphire substrate 10. Then, a buffer layer11 made of GaN is formed on the sapphire substrate 10 by a MOCVD method.For forming each layer, a trimethyl gallium (TMG) gas can be used as aGa source, and an ammonia gas can be used as an N source.

[0059] Then, an n-side contact layer 12 made of an n-type GaN layercontaining silicon (Si) as an n-type impurity, an n-type clad layer 13made of an n-type AlGaN compound crystal layer containing silicon (Si)as an n-type impurity and an n-type guide layer 14 made of an n-type GaNlayer containing silicon (Si) as an n-type impurity are consecutivelygrown. A monosilane gas (SiH₄ gas) can be used as an Si source, andtrimethyl aluminum (TMA) gas can be used as an Al source.

[0060] Then, an active layer 15 having a multiple quantum well structurethat is made by stacking Ga_(x)In_(1−x)N (in which X≧0) compound crystallayers having different compositions is formed on the n-type guide layer14. A trimethyl indium (TMI) gas can be used as an In source.

[0061] After the active layer 15 has grown, a p-type guide layer 16 madeof p-type GaN containing magnesium (Mg) as a p-type impurity, a p-typeclad layer 17 made of a p-type AlGaN compound crystal layer containingmagnesium (Mg) as a p-type impurity and a p-side contact layer 18 madeof a p-type GaN layer containing magnesium (Mg) as a p-type impurity areconsecutively grown on the active layer 15. A cyclopentadienyl magnesiumgas can be used as an Mg source.

[0062] Then, heat treatment is carried out, for example, with a hot airdryer substantially in the same manner as in Example 1. The heattreatment was carried out in aerial atmosphere as a heat-treatmentatmosphere at a heating temperature of 385° C. for a heating time periodof 84 hours (3.5 days). In this manner, the p-type impurity contained inthe p-type guide layer 16, the p-type clad layer 17 and the p-typecontact layer 18 is activated, and the electric resistance of each ofthese layers can be decreased.

[0063] Then, a resist layer is formed on the p-side contact layer 18such that the p-side contact layer 18 above a position where an n-sideelectrode 20 is to be formed is exposed. While the resist layer is usedas an etching mask, the p-side contact layer 18, the p-type clad layer17, the p-type guide layer 16, the active layer 15, the n-type guidelayer 14 and the n-type clad layer 13 are selectively removed to exposethe n-side contact layer 12. Then, the resist layer is removed, and forexample, a platinum (Pt) layer and a gold (Au) layer are consecutivelydeposited on the exposed p-side contact layer 18, to form a p-sideelectrode 19. Further, for example, a titanium (Ti) layer, an aluminum(Al) layer, a platinum layer and a gold layer are consecutivelydeposited on the exposed n-type contact layer 12, to form the n-sidecontact layer 20. Then, heat treatment is carried out to convert then-side electrode 20 to an alloy. In this manner, the semiconductor laser(LD) whose schematic cross-sectional view is shown in FIG. 4 can becompleted.

[0064] While the present invention has been explained with reference topreferred embodiments hereinabove, the present invention shall not belimited thereto. Conditions and various values explained in Examples andmaterials used therein are given for an explanation purpose and can bealtered as required. The method of forming each layer made of a nitridecompound semiconductor layer shall not be limited to the MOCVD methodand may be replaced with an MBE method, a hydride vapor depositionmethod in which a halogen contributes to transportation or a reaction,or other methods. Further, the sapphire substrate may be replaced with aGaN substrate or an SiC substrate. Further, while Example shows asemiconductor laser (LD) as a semiconductor device, the method ofproducing a semiconductor device, provided by the present invention, canbe applied to the production of a light-emitting diode (LED) ortransistors such as HBT.

[0065] In the method according to the first aspect of the presentinvention, the heating temperature is set at a temperature lower thanthat employed in any conventional method and the heat treatment iscarried out for a longer period of heating time period than thatemployed in any conventional method, whereby the resistance of a nitridecompound semiconductor layer can be decreased, and the nitride compoundsemiconductor layer can be activated. In the method according to thesecond aspect of the present invention, the heating temperature T andthe heating time period t are defined, whereby the resistance of anitride compound semiconductor layer can be reliably decreased, and thenitride compound semiconductor layer can be reliably activated. Further,in the method according to the third aspect of the present invention,the heat treatment is carried out in an atmosphere containing an oxygengas or containing steam, so that the lower-limit value of the heatingtemperature can be set at a low temperature as compared with anyconventional method. Further, the heating temperature is set at atemperature lower than 400° C., whereby the decomposition pressure of anitride compound semiconductor layer comes to be substantially zero, sothat the dissociation of nitrogen atoms from the nitride compoundsemiconductor layer can be reliably, prevented. Moreover, the heatingtemperature can be set at a temperature lower than that employed in anyconventional method, no or little diffusion of the p-type impurity suchas Mg takes place, or no or little disintegration of sharpness of aninterface takes place in a superlattice structure by diffusion of atoms(for example, In) constituting the nitride compound semiconductor layer,so that the active layer, for example, of a semiconductor laser (LD) isdeteriorated in no case or almost no case, whereby a high qualitysemiconductor laser (LD) can be produced. While it is required to carryout the heat-treatment for a long period of heating time period ascompared with a conventional method, further, processing can be carriedout in a large quantity at once so long as the heating temperature islower than 400° C., so that the method of the present invention cancomply with the mass-production of semiconductor devices. Further, whenthe heating temperature is lower than 400° C., no or little oxidationtakes place on the surface of a nitride compound semiconductor layeralthough it differs depending upon a heat-treatment atmosphere. When ap-side electrode is formed on the surface of a nitride compoundsemiconductor layer after the heat treatment of the nitride compoundsemiconductor layer, it is required to remove an oxide layer on thesurface of the nitride compound semiconductor layer. When the heatingtemperature is lower than 400° C., however, it is easier to remove theoxide layer formed on the surface of the nitride compound semiconductorlayer.

What is claimed is:
 1. A method of heat-treating a nitride compoundsemiconductor layer, comprising heating a nitride compound semiconductorlayer doped with a p-type impurity at a temperature that is at least200° C. but less than 400° C. for at least 100 minutes.
 2. The method ofheat-treating a nitride compound semiconductor layer according to claim1, in which the heating atmosphere is an aerial atmosphere.
 3. Themethod of heat-treating a nitride compound semiconductor layer accordingto claim 1, in which the heating atmosphere is an atmosphere supplied atleast with an oxygen gas.
 4. The method of heat-treating a nitridecompound semiconductor layer according to claim 1, in which the heatingatmosphere is an atmosphere supplied with an oxygen gas and a hydrogengas.
 5. The method of heat-treating a nitride compound semiconductorlayer according to claim 1, in which the heating atmosphere is an inertgas atmosphere or a reduced-pressure atmosphere having a pressure lowerthan atmospheric pressure.
 6. The method of heat-treating a nitridecompound semiconductor layer according to claim 5, in which the heatingatmosphere contains steam.
 7. The method of heat-treating a nitridecompound semiconductor layer according to claim 1, in which the nitridecompound semiconductor layer is heated for at least 200 minutes.
 8. Themethod of heat-treating a nitride compound semiconductor layer accordingto claim 1, in which the nitride compound semiconductor layer is heatedfor at least 500 minutes.
 9. The method of heat-treating a nitridecompound semiconductor layer according to claim 1, in which ahydrogen-permeable film is formed on the surface of the nitride compoundsemiconductor layer.
 10. A method of heat-treating a nitride compoundsemiconductor layer, comprising heat-treating a nitride compoundsemiconductor layer doped with a p-type impurity in a state where theheating time period t (unit: minute) and the heating temperature T(unit: K) satisfy conditions of t≧100 and the following equation (1),T≧α/[ln({square root}{square root over (t)})+ln(D ₀)−ln(C)]  (1) whereinα is a coefficient that is 1.04×10⁴, ln(D₀) is a coefficient that is 53and C is a carrier concentration (unit: cm⁻³) of the nitride compoundsemiconductor layer after the heat treatment thereof.
 11. The method ofheat-treating a nitride compound semiconductor layer according to claim10, in which the heating temperature T (K) is brought into a state wherethe condition of 473 (K)≦T<673 (K) is satisfied.
 12. The method ofheat-treating a nitride compound semiconductor layer according to claim10, in which the heating atmosphere is an aerial atmosphere.
 13. Themethod of heat-treating a nitride compound semiconductor layer accordingto claim 10, in which the heating atmosphere is an atmosphere suppliedat least with an oxygen gas.
 14. The method of heat-treating a nitridecompound semiconductor layer according to claim 13, in which the heatingatmosphere is an atmosphere supplied with an oxygen gas and a hydrogengas.
 15. The method of heat-treating a nitride compound semiconductorlayer according to claim 10, in which the heating atmosphere is an inertgas atmosphere or a reduced-pressure atmosphere having a pressure lowerthan atmospheric pressure.
 16. The method of heat-treating a nitridecompound semiconductor layer according to claim 15, in which the heatingatmosphere contains steam.
 17. The method of heat-treating a nitridecompound semiconductor layer according to claim 10, in which ahydrogen-permeable film is formed on the surface of the nitride compoundsemiconductor layer.
 18. A method of heat-treating a nitride compoundsemiconductor layer, comprising heating a nitride compound semiconductorlayer doped with a p-type impurity, at a temperature that is at least200° C. but not higher than 1200° C., in one atmosphere selected from;(A) aerial atmosphere, (B) an atmosphere supplied with an oxygen gas anda hydrogen gas, (C) an atmosphere supplied with an oxygen gas and steam,(D) an atmosphere supplied with an oxygen gas, a hydrogen gas and steam,(E) an inert gas atmosphere containing steam, or (F) a reduced-pressureatmosphere containing steam and having a pressure lower than atmosphericpressure.
 19. The method of heat-treating a nitride compoundsemiconductor layer according to claim 18, in which the nitride compoundsemiconductor layer is heated at a temperature that is at least 200° C.but less than 400° C.
 20. The method of heat-treating a nitride compoundsemiconductor layer according to claim 18, in which a hydrogen-permeablefilm is formed on the surface of the nitride compound semiconductorlayer.
 21. A method of producing a semiconductor device, including thestep of heating a nitride compound semiconductor layer, said stepcomprising heating a nitride compound semiconductor layer doped with ap-type impurity at a temperature that is at least 200° C. but less than400° C. for at least 100 minutes.
 22. The method of producing asemiconductor device according to claim 21, in which the heatingatmosphere is an aerial atmosphere.
 23. The method of producing asemiconductor device according to claim 21, in which the heatingatmosphere is an atmosphere supplied at least with an oxygen gas. 24.The method of producing a semiconductor device according to claim 23, inwhich the heating atmosphere is an atmosphere supplied with an oxygengas and a hydrogen gas.
 25. The method of producing a semiconductordevice according to claim 21, in which the heating atmosphere is aninert gas atmosphere or a reduced-pressure atmosphere having a pressurelower than atmospheric pressure.
 26. The method of producing asemiconductor device according to claim 25, in which the heatingatmosphere contains steam.
 27. The method of producing a semiconductordevice according to claim 21, in which the nitride compoundsemiconductor layer is heated for at least 200 minutes.
 28. The methodof producing a semiconductor device according to claim 21, in which thenitride compound semiconductor layer is heated for at least 500 minutes.29. The method of producing a semiconductor device according to claim21, in which a hydrogen-permeable film is formed on the surface of thenitride compound semiconductor layer.
 30. A method of producing asemiconductor device, including the step of heat-treating a nitridecompound semiconductor layer doped with a p-type impurity in a statewhere the heating time period t (unit: minute) and the heatingtemperature T (unit: K) satisfy conditions of t≧100 and the followingequation (1), T≧α/[ln({square root}{square root over (t)})+ln(D₀)−ln(C)]  (1) wherein α is a coefficient that is 1.04×10⁴, ln(D₀) is acoefficient that is 53 and C is a carrier concentration (unit: cm⁻³) ofthe nitride compound semiconductor layer after the heat treatmentthereof.
 31. The method of producing a semiconductor device according toclaim 30, in which the heating temperature T (K) is brought into a statewhere the condition of 473 (K)≦T<673 (K) is satisfied.
 32. The method ofproducing a semiconductor device according to claim 30, in which theheating atmosphere is an aerial atmosphere.
 33. The method of producinga semiconductor device according to claim 30, in which the heatingatmosphere is an atmosphere supplied at least with an oxygen gas. 34.The method of producing a semiconductor device according to claim 33, inwhich the heating atmosphere is an atmosphere supplied with an oxygengas and a hydrogen gas.
 35. The method of producing a semiconductordevice according to claim 30, in which the heating atmosphere is aninert gas atmosphere or a reduced-pressure atmosphere having a pressurelower than atmospheric pressure.
 36. The method of producing asemiconductor device according to claim 35, in which the heatingatmosphere contains steam.
 37. The method of producing a semiconductordevice according to claim 30, in which a hydrogen-permeable film isformed on the surface of the nitride compound semiconductor layer.
 38. Amethod of producing a semiconductor device, including the step ofheating a nitride compound semiconductor layer doped with a p-typeimpurity, at a temperature that is at least 200° C. but not higher than1200° C., in one atmosphere selected from; (A) aerial atmosphere, (B) anatmosphere supplied with an oxygen gas and a hydrogen gas, (C) anatmosphere supplied with an oxygen gas and steam, (D) an atmospheresupplied with an oxygen gas, a hydrogen gas and steam, (E) an inert gasatmosphere containing steam, or (F) a reduced-pressure atmospherecontaining steam and having a pressure lower than atmospheric pressure.39. The method of producing a semiconductor device according to claim38, in which the nitride compound semiconductor layer is heated at atemperature that is at least 200° C. but less than 400° C.
 40. Themethod of producing a semiconductor device according to claim 38, inwhich a hydrogen-permeable film is formed on the surface of the nitridecompound semiconductor layer.